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Post by Scottish Cartie Association on Jul 22, 2011 15:33:37 GMT
As some of you will know, the Scottish Cartie Association's programming elves have been hard at work building a computer program to estimate cartie speeds on any given track. It is now in its final stage of testing and should be available for download some time next month. In the mean time, have a look at some of the graphs produced at scottishcarties.org.uk/news/2011/cartieSimComingSoon#comment-2684.
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Post by Scottish Cartie Association on Aug 1, 2011 11:51:07 GMT
SCA CartieSim version 1.0 is now available for download from scottishcarties.org.uk/cartiesim/download. Features include; - Course data loaded from GPS file, CSV file or online from Google Maps and satellite altitude database
- Simple "Course Wizard" guides you through loading a new course
- Calculates speed profile, time and maximum speed
- Calculates course data such as total length, average gradient, maximum gradient
- "Critical Velocity" calculation predicts sections where braking may be required
- Models dead, push and ramp starts
- Models different cartie configurations for comparison
- Assess the effect of adding chicanes
- Calculates stopping distance after finish line
- Export course profile and map to image file (licensed version only)
- Free download
- Course profiles for Cairngorm, Castle Forbes, Border Bogies, Dalby etc included
Chart showing predicted speeds against GPS data logged by Night Train at CSEx2011. Chart showing the effect of ballast, with all other parameters kept constant.
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Post by Organgrinder on Aug 1, 2011 16:05:46 GMT
Interesting to see that the lighter carties don't seem to be able to brake later or carry any more speed through the bends. Given the laws of physics, that really surprises me - or are we to assume that the driver of the light cartie is, after all's said and done, a Jessie?
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Post by Scottish Cartie Association on Aug 1, 2011 17:38:35 GMT
It doesn't model braking on the course to that level of accuracy. Only after the finish line does it properly apply a decceleration through braking, as that's the only time you know the braking point. Elsewhere, it'd involve a fairly horrible iterative algorithm.
Also - mass has no effect on max cornering speed, or - more correctly - the effect cancels out. Cornering force is Mass x speed x speed / corner radius. Friction of tyre needs to match this, and is given by mass x static friction coefficient. The mass term on both sides cancels out.
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Post by Scottish Cartie Association on Aug 2, 2011 10:10:29 GMT
Hmm - I think I can probably explain that better.
For a cartie of mass m travelling around a bend of radius r at a speed v;
Cornering (centripetal) force Fc = mv2/r
The tyres must grip to keep the cartie going around the bend. The cartie's grip (static friction) is given by Cfmg cos(inc), where Cf is the coeffiction of static friction, g is the acceleration due to gravity and inc is the angle of the slope (you have less grip on steep hills).
The maximum speed is when cornering force equals the maximum cornering grip. i.e.
mv2/r = Cfmg cos(inc)
The mass term on both sides cancels out, so;
v = ( Cfgr cos(inc) )1/2
i.e - maximum cornering speed for a cartie is independant of the cartie's mass.
The only ways to increase cornering speed are to;
1) Increase Cf (i.e. get stickier tyres) 2) Increase g (i.e. race on saturn) 3) Increase the normal force on the tyres without changing the mass (i.e. aerodynamic downforce) 4) Stop being such a massive jessie
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Post by Scottish Cartie Association on Sept 26, 2011 8:52:56 GMT
A new version of SCA CartieSim has just been released. There are numerous tweaks and bug fixes in it, but the main changes are; * New "Quick Edit" tool which allows you to quickly see the effect of changing cartie weight, rolling resistance, CoG height etc. * Direct import of excel files created by Trackmaster. If you've logged an actual race as a single session, you'll see each run overlayed on the speed profile. * Calculation of braking points and deceleration under braking for tight corners and chicanes. The latest version can be downloaded from here. The screenshot below shows data logged for the Sideweasel's three runs at Belchford this year, overlaid on the calculated speed profile in green.
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Post by neils on Sept 27, 2011 7:58:31 GMT
Would it be possible to put one of the european tracks into your software? I suppose you need more info on location first
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Post by Scottish Cartie Association on Sept 27, 2011 8:31:00 GMT
Absolutely. It'll import a route from google maps, google earth, satnav or gps data files.
If you know the start point and end point, set up a route on google maps (right click and drop "start from here" and "end here" markers, then copy the url of the route (top right of the map) and paste it into the new course wizard.
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Post by zonagravedad on Sept 27, 2011 10:43:00 GMT
One small detail about influence of mass in bend speed. The increase in tire grip is not linear with the increase in mass because of the tire load sensitivity. As mass increases, the coefficient of friction of the tire, and hence its maximum grip, decreases. That implies two things: - Everything else being equal, the same cartie has a higher max. speed through a bend with less weight. - The lateral load transfer ( function ot center of gravity height and track width ) takes load from the inner wheels and translates it to the outer tires. Why the total grip is less if the total load is the same ? Because of the decrease in coefficient of friction of the outer tires ( which now support most of the cartie weight ).
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Post by Scottish Cartie Association on Sept 27, 2011 11:34:41 GMT
True. But is it not close enough over the ranges we're talking about? If you're planning a race and want to know the points where a loss of control is likely it works well enough.
Do you have a practical formula to give coefficient of friction in terms of m?
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Post by zonagravedad on Sept 27, 2011 13:50:57 GMT
If that formula would exist it would be like the philosophal stone... Maximum lateral acceleration depends on a lot of factors: - Tires: Compound ( friction coefficient ), contact path ( size and orientation ), camber angle ( static, roll induced camber, steering induced camber ), pressure, temperature, carcass type ( bias, radial ), cornering stiffness... - Suspension: % time of tire contact to ground, spring stiffness, compression damping, rebound damping, anti roll bars... - Vehicle geometry: Center of gravity height and fwd-aft , track width, wheelbase, roll centers, steering geometry, camber, caster and kingpin angles and its variations with wheel travel and/or steering. Oversteer, understeer neutral steer tendency. - Others: Aero download, brake bias, driving style, slip angle... - Etc., etc.
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Post by Scottish Cartie Association on Sept 27, 2011 14:31:50 GMT
Quite. But my initial point still stands - over the range of weights we are dealing with - i.e. somewhere between 150 and 200Kg, I suspect that the effect of load sensitivity is negligible.
In your graph the load increase to go from coefficient of friction of 1.1 to 0.9 is 900lbs (408Kg). Over the 50Kg variation likely for carties, doesn't that equate to about +/- about 0.01?
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Post by zonagravedad on Sept 28, 2011 7:12:48 GMT
Well, this graph is for a car tyre designed for a 1500 Kg vehicle, and a bicycle tyre is designed for some 100 Kg weight, so I suppose that load sensitivity will be quite different, but I ave never seen such information.
As extra weight is OK to overtake aero drag and to keep momentum in low slope sections, I guess that the right setup ( and the one that wins European Championships year after year ) is to keep weight to the maximum allowed and adjust the rest of parameters.
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